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Number of hops can be reseted!!!!!!!!!!

Hugo Tigre
Trusted Contributor

Number of hops can be reseted!!!!!!!!!!

This doesn't make much sense to me, but i read somewere that a layer 3 switch can reset the number of hops in a packet (ttl) as the packet passes through the switch.

Does anyone know if this is accurate, i'm thinking no, but i'm not shure.

thanks in advance

regards,
Hugo
5 REPLIES
Les Ligetfalvy
Esteemed Contributor

Re: Number of hops can be reseted!!!!!!!!!!

Just because a switch is layer 3 capable does not mean traffic will go on layer 3.
Hugo Tigre
Trusted Contributor

Re: Number of hops can be reseted!!!!!!!!!!

so what u r saying is that, this is not accurate, and the switch doesn't reset the hop count?
Les Ligetfalvy
Esteemed Contributor

Re: Number of hops can be reseted!!!!!!!!!!

No, I am not saying that. Since a layer 3 switch can do both layer 2 or layer 2, you need to clarify.

If the switch forwards a packet on layer 3, then the TLL would be decremented. Layer 2 would not.

Why not setup a lab and sniff the traffic and see for yourself?
Ardon
Trusted Contributor

Re: Number of hops can be reseted!!!!!!!!!!

Hi Hugo,

Can you quote what you read? Or maybe it stated that every IP Router Interface decrements the TTL in the IP Header by 1 which is exactly what happens.
The function of the TTL field is easy. It's to prevent IP Packets to travel around in the network continuously in case there is a Layer 3 loop where for example Router A points to Router B for network X. Router B in turn points to Router A for network X.
Without a TTL decrementation every time it hits a router interface the packet(s) would travel between Router A and Router B the whole time. As more packets enter this looping state it will eventually congest the link. However WITH a TTL field that decrements eventually the "TTL will expire in transit" . The packet will be dropped when the last router decrements the TTL to zero. So it does not make any sense indeed to "reset" the TTL.

Regards, Ardon
ProCurve Networking Engineer
Les Ligetfalvy
Esteemed Contributor

Re: Number of hops can be reseted!!!!!!!!!!

Also releated to what's known as quantum packet accelerator (or blinkenlights) on layer 2. If TTL decrementation were true on layer 2, we would not have to worry so much about STP.

Usually the TTL on arriving data is not so close to exiry as to be of concern except in some cases. I know some ISPs will mess with the TTL, setting it to 1 so that if you put in a DSL router to try and *share* your internet connection, the packets die at the router.